A backlight is a form of illumination used in liquid crystal displays (LCDs). Backlights illuminate the LCD from the side or back of the display panel, unlike front lights, which are placed in front of the LCD. Backlights are used in small displays to increase readability in low light conditions and in computer displays and LCD televisions to produce light in a manner similar to a cathode ray tube (CRT) display. The typical LCD backlight has traditionally been cold cathode fluorescent lamps (CCFLs). Increasingly, however, light-emitting diodes (LEDs) are becoming the predominant backlighting technology of choice.
Currently, LED backlighting is most commonly used in small, inexpensive LCD panels. Recent advances in LED technology, packaging, and materials have provided dramatic increases in LED brightness which, in turn, has led to the use of the LED in all types of lighting applications, including LCD backlights. The challenge for LED backlights is to get the heat out of the LED device itself and then out of the display assembly. Other than the performance of the LCD, back-lighting is often the most important technology affecting display image quality. Although incandescent backlighting can be used when very high brightness is desired, the use of incandescent bulbs has many drawbacks, such as limited life and the amount of heat generated, which often means that the bulb needs to be mounted away from the display. Over the last several years, small color LCD displays have been integrated into an ever-broadening range of products. Color displays, once considered a luxury in electronic products, are now a standard offering even at the entry level. Fortunately, the economies of scale have lowered the cost of LCD color displays making them attractive for integration in many different types of electronic products and instruments.
Color LCD displays typically require a white backlight for proper viewing in any lighting environment. This backlight subsystem most often consists of an array of high brightness white LEDs, a diffuser to distribute the light, and a backlight driver to convert the available power into regulated constant current to drive the LEDs. The size of the display will often determine the number of LEDs required for its backlighting. For LEDs, the light output is proportional to current, and since LEDs have a very steep I-V curve it is important that the current through the LEDs be closely matched to ensure even lighting since LEDs are typically distributed across one edge of the LCD display. In addition, software control is necessary so the user can adjust the brightness and compensate for ambient light conditions. The color point of the LED can shift depending on the current through the LED, so it is common to set the LED current to a fixed value and pulse-width modulate the LEDs to reduce the average light output. There are a number of factors that need to be considered when incorporating a small color LCD display into a design to achieve the right balance of cost and performance.
Small LCD displays have also been used in connection with automotive applications, such as in an automotive mirror. Applications illustrating various types of automotive mirror displays are shown in U.S. Pat. Nos. 6,870,655; 6,737,630; 6,572,233; 6,552,326; 6,420,800; 6,407,468; 6,346,698; 6,170,956; 5,883,605; and 5,825,527, U.S. patent application Ser. No. 12/193,426 entitled “Vehicle Rearview Assembly Including A Display for Displaying Video Captured by a Camera and User Instructions,” and U.S. patent application Ser. No. 12/196,476 entitled “Discrete LED Backlight Control for a Reduced Power LCD Display System,” all commonly assigned to Gentex Corporation and all of which are incorporated herein by reference in their entireties. A common example of a video display located directly in an auto dimming rearview mirror is when it is paired with a rear camera display (RCD). In this application, the display shows a real-time panoramic view of the rear of the vehicle. The LCD display automatically appears through the mirror glass when the vehicle is shifted into reverse. The display disappears when the vehicle is shifted into any other gear. In operation, a 60 mm LCD or the like appears through the mirror's reflective surface. The result is a bright, high-resolution display in an intuitive location useful to the driver.